Siemens MMT 500 AC/AC drive systemAC drive technology yields 6-7% higher efficiency than DC drives at higher power levels and provides significant improvement in control functions. This technology means no power contactors or degradable components. Except for the periodic bearing lubrication, the system is inherently maintenance-free. Power slowing to standstill all but eliminates wear on the mechanical service brakes.The traction alternator converts the mechanical energy of the diesel engine into DC current, charging the DC bus capacitors. The electric power passes through the capacitors to two inverters, which produce AC current for the traction motors. The output frequency and voltage of the inverters is controlled to provide precise motor torque and speed.In retardation mode, the motors apply braking force by generating electricity which is converted to DC by the inverters. Braking choppers, connected to the inverters, channel that power straight into a resistor grid that continuously dissipates the energy until the truck reaches standstill. So braking is smooth, like driving a car, but without mechanical break wear. (Source: Siemens)